The objective of the research contained in this application is to pinpoint specific structural features which control the rates and efficiencies of the various photochemical processes which take place in photosynthetic complexes. The present proposal focuses on carotenoids which are essential for the survival of photosynthetic organisms. The systems to be studied are photosynthetic bacterial reaction center and antenna pigment-protein complexes which have been isolated, pigment-reconstituted, crystallized and genetically altered. X-ray crystallography and NMR spectroscopy will be used as direct structural probes. Optical and electron paramagnetic resonance spectroscopy will allow the correlation of structure with spectroscopic observables and function. In turn the relationships of the spectroscopic observables to the structures may be extended to interpret spectroscopic studies on systems not yet crystallized, thus providing a basis from which to understand more complex systems; e.g. higher plant and algal protein complexes. The specific aims of this proposal are: (1) To determine the structures of carotenoids in photosynthetic pigment-protein complexes; (2) To determine the relationships between the carotenoid structures and their spectroscopic properties; and (3) To determine the relationships between the carotenoid structures and their mechanisms of carotenoid-to-bacteriochlorophyll singlet and bacteriochlorophyll-to-- carotenoid triplet energy transfer. Several feasibility studies have already been carried out in support of the methods proposed to be employed.